82 EXPLORATIONS IN TURKESTAN. 
It therefore does not seem advisable to adduce the Tian Shan ranges in ilhistra- 
tion of the direct action of a " tanefcntial movement," as has been done by Sness 
(1897, i, 619) on the basis of Mnshketof's description. It is tnily 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 
fonnation, 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 be emploj-ed as the base of further theoretical 
considerations. 
THE BEARING OF THE TIAN SHAN RANGES ON THE THEORY OF HORSTS. 
The Bural-bas-tau and its fellows deser\'e special consideration in connection 
with the theor)' of the origin of horsts, or upstanding crustal blocks, as set forth 
by Sness, 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 pennanently 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 stationar)- ; 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 faxdt planes b}- 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 abimdant 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 sup- 
position 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 jost- 
ling and uplift within a surrounding region of relative stability, nor the counter 
