﻿370 THE REV. J. F. BLAKE ON THE [May I905,. 



rock (sandstone, tuff or igneous), only to be matched in the Series 

 itself. 1 



Regarding the second criterion of an autoclastic rock, that they 

 will contain fragments of the overlying rock 2 : this involves our 

 knowing which is the overlying rock, and thereby the order of 

 succession. If the order of succession were that adopted in the 

 Geological Survey-Memoir, it would be easy enough to prove that 

 this criterion was satisfied ; and one only wonders why no use was 

 made of this in arguing for a ' crush-conglomerate.' But, when 

 the order of succession is reversed, as in the present instance, the 

 three uppermost strata have in places an obscure colour-banding, 

 which is somewhat difficult to distinguish in a fragment from the 

 lamination exhibited by beds below both the Schistose Breccia and 

 the Barrule Slate. 



Regarding the third criterion, that an autoclastic rock may 

 generally be found graduating into a brecciated or semi-brecciated 

 form : although there are numerous examples, I have found none 

 graduating into anything else than their original form. Sulby 

 Crags are no exception to this ; the nearest breccias are all of one 

 kind, they are separated from the Schistose Breccia by a well- 

 marked line, and the adjoining contorted flags are only bent, not 

 broken ; moreover, this is not the direction in which the structure 

 is continued. The only case somewhat resembling an exception is 

 the Hill Series west of Snaefell : on tracing the rounded sandstone- 

 like pebbles from Ramsey up the strike to the Hill Series, we should 

 find there such a majority like them, that we cannot be far from the 

 centre whence this rock was dispersed; but these are mixed, at 

 Ramsey, with many other varieties, which would prove too much. 



From the failure of all the special characters of an autoclastic rock, 

 I think we may conclude that none of those enumerated are of that 

 character; the fragments which they contain have never been brought 

 to their present position by two rocks shearing together under 

 pressure, but consist rather of ' rock of an original fragmental 

 character,' rendered schistose, and crushed out sometimes into 

 smaller fragments, of which the cause is obvious in their later 

 contortions. 



There are also some peculiarities about the Schistose Breccia 

 which carry strong weight in the same direction. (1) To call the 

 rock a ' conglomerate ' is to convey a wrong impression : it is very 

 seldom that any of the fragments are rounded ; the problem is 

 rather to realize how they may possibly be removed from their 

 original home with so little change, even with the freest motion, 



1 It may be noted in this connection that Prof. Watts, who assisted in the 

 establishment of the term crush-conglomerate, writing three years later in 

 his ' Geology for Beginners ' (1898) says that they will differ from those depo- 

 sited by water-action in three ways, of which No. 3 is ' the pebbles are all 

 made of one kind of rock,' while a ' thrust-conglomerate will not differ much 

 from a crush-conglomerate, except that two or three types of fragments may be 

 present in it.' (Op. cit. p. 111.) 



2 See also J. E. Marr, ' Principles of Stratigraphical Geology ' 1898, p. 81. 



