62 Analyses of Books. Jury, 
about the origin of the crystals of schorl, &c. They have some- 
times the appearance of being broken, and sometimes are incur- 
vated. I conceive the broken appearance is merely a deception 
arising from the simultaneous formation of various crystals, which 
are intermixed with each other. I have seen similar appearances in 
the crystallization of various mixtures of salt in my own laboratory. 
The curve must also have been the original state of the crystal which 
Dr. M. describes. I recollect several years ago to have obtained, 
crystallized in a common phial, large curved crystals of muriate of 
strontian. The crystal of schorl passing through garnet must be 
explained on similar principles. 
Craig Cailleach, a mountain near Killin, is composed of chlo- 
tite-slate, and contains veins and nodules of quartz. In this quartz 
crystals of rutile occur. 
Dr. M. enters into some speculations respecting the contortions in 
the strata of mica-slate visible at Loch Lomond, and infers that they 
must have been produced by the action of external forces. The 
reasoning of the Huttonians on this subject is well known. The 
subject ought to be considered in a more general point of view than 
has hitherto been done. Nothing is more usual than to find granite, 
gneiss, marble, lime-stone, green-stone, basalt, &c. composed of 
granular distinct concretions. A section of all such rocks would 
yield the same appearance of contortions. ‘The apparent contor- 
tions in mica-slate, and in the contemporaneous veins which Dr. 
M. has drawn, are owing to the same cause, whatever it is, that has 
produced the granular distinct concretions in the above-mentioned 
rocks. It is impossible to ascribe this to an external force. It 
must be owing to a law connected with the original formation of the 
stone similar to that which produces the various cleavages in crys- 
tals, and depending obviously upon the same cause. 
Dr. M.’s observations on grey-wacke and transition slate appear 
to me perfectly just ; though I am not aware that they possess any 
novelty. They may, however, be useful in drawing the attention 
of English mineralogists to the definition of a term which they are 
in the habit of using with too much latitude. I do not think that 
the term mechanical, in the usual sense of the word, can be applied 
to grey-wacke. 1 believe it was originally formed in the same state 
as it exists in at present, and that it is not a true sand-stone. 
Our author terminates his paper with an account of the rocks 
about Aberfoyle and Loch Ketterin. He considers them as grey- 
wacke alternating with mica-slate. As far as I could make out these 
rocks, for I have been on the spot, they are primitive ; sometiries 
mica-slate, sometimes quartzose clay-slate. There can be no 
doubt that the primitive rocks graduate imperceptibly into the tran- 
sition ; but so do the transition into the floetz, as may be seen very 
well in Berwickshire; so that if this gradation were sufficient, as 
the Doctor supposes, for inducing us to confound the primitive 
and transition rocks, it would be equally sufficient to induce us to » 
