SKY. GEOLOGY. SECONDARY STRATA. 339 



The examination of these strata in Trotternish is a task 

 of no small difficulty; in which, conjecture, or at least 

 analogy, must often supply the place of observation.* 

 This difficulty is caused by the trap which covers them 

 more or less completely throughout their whole extent, 

 whence they exhibit on the surface but occasional and 

 slender indications of their nature, or even of their 

 existence; a precipitous rock, or the section afforded 

 by a stream, bringing a small patch or a few scattered 

 fragments to light. It is only from the shore line, where 

 the sections found in the high cliffs are continuously 

 displayed, that a notion can be formed of their extent 

 or dimensions ; and even those appearances are so much 

 involved and obscured by the trap connected with them, 

 that they rarely allow the order of the strata to be traced ; 

 while their inaccessible state often compels us to judge 

 of their composition by indications which ought never 

 to be trusted where actual contact can be obtained, 

 It may be added to these insuperable causes of obscurity, 

 that even the natural order of the strata is inconstant 

 and uncertain, and does not therefore afford that assistance 

 which might, in conjunction with the accessible indi- 

 cations, allow us to ascertain the real order of their 

 arrangement. 



In examining this coast some assistance will be derived 

 from a previous acquaintance with Rasay.f Other ana- 

 logies will also readily suggest themselves to English 

 geologists : if Scotland contains any similar collections 

 of strata from which more convenient illustration might 

 be derived, they remain still concealed to stimulate the 

 industry of future observers in that neglected country. 



Under the circumstances of obscurity and want of 

 connexion in which the strata of Trotternish appear, 



of substances, but a brief and convenient mode of reference for topo- 

 graphic details that would either require tedious descriptions, or, for 

 want of geographic marks, be unintelligible. 



* PL XIV. fig. 1. f PL XV. 



