426 ARRAN. MINERALS. 



gous appearances on a much larger scale in the prehnite 

 of Dumbarton and Renfrew ; and as these circumstances 

 have not hitherto been investigated, it will be useful to 

 describe them as they occur in those places. 



A very extensive district of trap rocks is here found 

 occupying both shores of the Clyde ; conspicuous for 

 the quantity of prehnite which it contains ; that mineral 

 being associated, as is not unusual, with many more 

 of the zeolitic substances that appertain to this rock. 

 The prevailing are, stilbite, nadelstein, and analcime: 

 laumonite and ichthyophthalmite are more rare, and it 

 is doubtful if mesotype exists ; as it is occasionally im- 

 possible, without chemical examination, to distinguish 

 this mineral from nadelstein when it occurs in minute 

 nodules and passes to the opake state. In certain places, 

 harmotome abounds, and it is scarcely necessary to add, 

 that the nodules of quartz and of chalcedony so common 

 in the trap rocks of Scotland, are also found in this. 

 It is unnecessary to notice the other imbedded minerals, 

 as they are not connected with the object at present 

 in view. 



The prehnite here exhibits three very distinct varieties 

 of texture, or of aspect; being fibrous, lamellar, or pre- 

 senting an even fracture which is either flat or imperfectly 

 conchoidal. It is rarely crystallized ; the most common 

 form being that of a solid mass, with a tubercular or bo- 

 tryoidal surface in those places where the cavity which 

 it occupied has been partially empty. These are at times 

 so accumulated as to present stalactitical forms. Occa- 

 sionally however, the surfaces of these tubercles are echi- 

 nated with crystalline terminations, generally very obscure, 

 but sometimes sufficiently distinct to give indications, at 

 least, of the geometric forms which they seem endeavour- 

 ing to assume. In a few rarer instances, independent 

 but minute crystallizations may be observed; but the 

 forms even of these are confused, either from aggrega- 

 tion, extreme minuteness, or want of geometric decision. 



