Crystalline Structures of Rocks. 71 



mate to the trap ; while their substance, at the same time, is con- 

 verted into an anomalous stone resembling those cherts which have 

 been sometimes called hornstone. An inequality of the internal 

 texture is here also ascertained by the botryoidal surface which 

 these assume on exposure to the sea. 



It is now important to remark, that these spherules, wherever 

 the forms are most perfect, present a concretionary structure, pass- 

 ing into one which is decidedly crystalline. Brilliant fibres radiate 

 from the centre, and are repeated at intervals, so as to form suc- 

 cessive concentric crusts of the same nature ; or else these crystal- 

 line spheres are surrounded with crusts, in which no fibrous struc- 

 ture can be traced. There is thus a transition from the most per- 

 fect crystalline to the most imperfect concretionary spherule. 



In attempting to explain these appearances, it is striking to ob- 

 serve how these spheroidal crystallized forms coincide with those 

 which occur in glass under certain circumstances, and how accu- 

 rately they resemble the analogous appearances produced in 

 Mr. Watt's well-known experiments. In these it was remarked, 

 that the crystalline arrangement was enabled to proceed after the 

 fused trap had lost its fluidity. Thus it is equally easy to com- 

 prehend how a solid mass of any or the above-named rocks, soft- 

 ened, if it is necessary to suppose so, without fusion, or otherwise 

 under the long- continued influence of heat, might have assumed 

 a similar species of structure. As also in one of these cases there 

 is a gradual progress from the most perfect crystalline to the most 

 imperfect concretionary arrangement, there can be no reason to 

 doubt that, in every case, the latter may also be produced by the 

 same causes. It should lastly be added, in confirmation of this 

 theory, that a spheroidal structure of a similar nature exists in the 

 trap of the Shiant Isles. 



A spheroidal structure, terminating also by wasting in botryoidal 

 forms, has been observed in certain limestones, as in that of Sun- 

 derland. A similar arrangement is occasionally found in the sand- 

 stones; and sometimes, in the red varieties, it is indicated by the 

 presence of white spheroidal spots. Of these no explanation has 

 been suggested, and I have none to offer. 



