Crosby.] a4 [January 21, 
of great pressure and a more or less elevated temperature. Of the 
solvent power of water under these conditions, few will need to be 
informed, and it is easy to guess what part of the rock would be 
most affected by it; viz., the cement, for this is not only more easily 
reached by the water, but it is silica which has been deposited from 
aqueous solution, and is probably not yet entirely anhydrous, and 
consequently retains in a measure its capacity for dissolving in 
water. Whether the forces invoked are mechanical or chemical, or 
both, I think it is easy to see how the integrity of the cement may 
be destroyed, and the forms of the pebbles altered, while the solid 
grains of vitreous quartz remain essentially intact. Supposing that 
this happens, what chance is there for the development of fluidal 
structure? Take a ball of sand; so far as any definite arrangement 
of the particles is concerned, it may be said to be structureless. 
Impress it with the thumb and produce a slight distortion of its 
form. What has happened internally? a general displacement of 
the particles in that part of the ball, no doubt, but has any definite 
arrangement been established ? I think the microscope would fail to 
show it. We have simply disturbed the structure of a structureless 
mass! And yet I fail to see that the conditions are necessarily essen- 
tially different, when we pass from this ball of sand to a pebble of 
quartzite. Plasticity of the constituent particles, probably results 
in many, but not necessarily in all, cases of distortion. This part of 
the subject may be closed with the remark that the well-known 
facts with regard to the contortion without rupture of rigid rocks 
like quartzite and limestone, and the distortion of fossils in cleaved 
slates, etc., seem alone sufficient to place the distortion of pebbles in 
the catalogue of phenomena which are inherently probable. 
The conglomerate of the Boston basin is, with scarcely any 
doubt, of Primordial age, representing the same general horizon as 
the Braintree and Cambridge slates, although mainly underlying 
these. The evidence that some of the hardest pebbles in this 
conglomerate have been distorted by pressure is rather scanty, as 
the preceding discussion shows, and yet it seems to me fairly satis- 
factory. But I now invite the attention of the Society to a much 
older conglomerate in portions of which nearly all the pebbles have 
suffered an extreme degree of deformation, the evidence in this 
case being, apparently, all that could be desired, both as to quantity 
and quality. 
