82 EXPLORATIONS IN TURKESTAN.
It therefore does not seem advisable to adduce the Tian Shan ranges in illustration of the direct action of a " tangential movement," as has been done by Suess (1897, i, 619) on the basis of Mushketof's description. It is truly possible that compression may have had some share in producing the existing disordered attitude of the mountain blocks, for folds of late or post-Tertiary date occur in the Narin formation, as is further stated below ; but the share that compression had in raising the block ranges is so problematic that it should not to-day be accepted as an established fact, and still less should it he employed as the base of further theoretical
THE BEARING OF THE TIAN SHAN RANGES ON THE THEORY OF HORSTS.
The Bural-bas-tau and its fellows deserve special consideration in connection with the theory of the origin of horsts, or upstanding crustal blocks, as set forth by Suess, who regards such horsts as stationary parts of the earth's crust, with respect to which the-surrounding lower land has sunk (1897, i, 263, 774, 777, 782)• The evidence for this conclusion is chiefly that " we know of no force whatever that is capable of uplifting from below, between two plane surfaces, large or small mountainous masses, and of maintaining them permanently in such a situation, against the action of gravity" (1897, i, 782 ; also 775). This conclusion and the reason for it both seem to me to place too high a value upon what we do not know. It is, of course, conceivable that horsts have stood still while the surrounding lands have sunk down, but it is also conceivable that the horsts have been raised, while the surrounding lands have remained stationary ; that the horsts have risen and the surrounding lands have sunk ; and that both have risen, the horsts more than the rest, under conditions suggested by the citation from Gilbert, above made. The last supposition seems eminently applicable to the Tian Shan. Direct observation seldom, if ever, furnishes evidence by which one can choose among these various mechanical possibilities. In the case of the Tian Shan there is certainly not enough now known concerning the attitudes of the fault planes by which various blocks are divided to make it worth while to discuss this recondite aspect of the problem. As to the way in which blocks of the earth's crust might be dislocated into irregular attitudes, we can conceive of many theoretical processes, every one of which is permissible in the presence of our abundant ignorance of the constitution and behavior of the earth's interior. It seems, therefore, unsafe to-day to exclude all other processes than direct-acting gravity from a share in the production of horsts. Forces of uplift are still worthy of consideration. In such a problem it seems better to open the mind as freely as possible to reasonable speculation, rather than to restrain its inventive powers. Deep-seated movements of the earth's core, possibly due to deep-seated compression, may cause local internal up-swelling, over which the heavy-lying crust is broken and irregularly jostled in mountain blocks. It is this supposition that is entertained in Gilbert's suggestion as to the origin of the Basin ranges of Utah and Nevada, above cited ; but neither the supposition of local jostling and uplift within a surrounding region of relative stability, nor the counter